Devices and methods to enhance accuracy of torque sensors

ABSTRACT

This invention concerns torque sensor systems and methods that computationally compensate in real-time for hysteresis in signals output from sense elements that are indicative of a torque, including a time-varying torque. In preferred embodiments, temperature effects can also be compensated for by such methods and systems.

RELATED APPLICATIONS

This application claims the benefit of and priority to PCT applicationserial number PCT/US2016/037548, filed 15 Jun. 2016 and published asWO/2016/205313 on 22 Dec. 2016, which claims priority to U.S. patentapplication Ser. No. 14/741,305, filed 16 Jun. 2015, now U.S. Pat. No.9,435,708, issued on 6 Sep. 2016.

FIELD OF THE INVENTION

The present invention relates to devices and methods for correctingerrors in torque sensor systems. Specifically, the present inventionrelates to devices and methods of applying computational compensation toelectronic signals to overcome errors including, but not limited to,hysteresis inherent in torque sensor systems.

BACKGROUND OF THE INVENTION

The following description includes information that may be useful inunderstanding the present invention. It is not an admission that anysuch information is prior art, or relevant, to the presently claimedinventions, or that any publication specifically or implicitlyreferenced is prior art.

BACKGROUND

Basic to the operation of modern machinery is the transmission ofmechanical energy from source locations to points of utilization throughrotating shafts transmitting torque. Thus, in the control and monitoringof systems having rotating shafts, torque is a fundamental parameter ofinterest. Therefore, the sensing and measurement of torque in anaccurate, reliable, and inexpensive manner has been pursued for severaldecades.

Torque measurement has been accomplished using contact and non-contacttype sensors. One type of sensor that is in contact with a rotatingshaft is a “strain gauge”-type torque detection apparatus, in which oneor more strain gauges are directly attached to the shaft or hub carryingtorque. Strain on the shaft is translated to the strain gauge, whichcauses a change in resistance in the strain gauge that is typicallymeasured with a bridge circuit. As the sensor has to be directly incontact with the shaft under torque, both wired and wireless telemetrysystems have been developed to supply power to the strain gauges on theshaft as well as extract signals from them. Contact-type sensors tend tobe commercially impractical for use in many applications for a number ofreasons: (i) they tend to be relatively expensive; (ii) systems thatprovide wireless telemetry capabilities typically require a significantamount of volume in near proximity to the shaft which makes locatingthem in tight enclosures, such as gearboxes, difficult; (iii) they mayemit or be susceptible to electromagnetic emissions; and (iv) as thestrain gauges and the associated electrical components are located on orwithin the shaft transmitting the torque, there are also limitations asto the maximum rotational speed of the shaft permitted due to thecentripetal forces created, as well as limitations as to the maximumallowable temperature of the shaft. One type of non-contact torquesensor uses the magnetostrictive properties of a ring attached to ashaft carrying torque. See, e.g., U.S. Pat. Nos. 5,351,555 and5,520,059. Tensile “hoop” stress in the ring, associated with how thering is attached to the shaft, establishes a dominant, circumferentiallydirected, uniaxial anisotropy. Upon the application of torsional stressto the shaft, the magnetization reorients and becomes increasinglyhelical as torsional stress increases. The helical magnetizationresulting from torsion has a circumferential component as well as axialand radial components, with the magnitude of the axial componentdepending entirely on the torsion. The radial component will bedependent on torsion, but may also be influenced by other stressesapplied to the shaft such as bending (Garshelis & Tollens, 2010). One ormore magnetic field vector sensors can be used to measure the magnitudeand polarity of the magnetic field arising as a result of the appliedtorque in the space about the transducer in order to provide a signaloutput reflecting the magnitude and polarity of the torque. While thefields that arise from the ring itself have only hard axis componentsrelative to the anisotropy, “parasitic” fields from permeable materialthat is close enough to become magnetized by the ring field have no suchlimitation. The addition of such parasitic fields to thetorque-dependent field from the ring can seriously degrade thenear-ideal features of the transfer function (defined as the ratio ofthe output to input) of the measured magnetic field versus appliedtorsional stress to the shaft. In order to avoid a major source of suchdistortion, it is preferred that the shaft the ring is placed on befabricated from a paramagnetic material.

The elimination of issues associated with such ring constructionsspurred development of magnetoelastic torque transducers in which one ormore active, torque-sensing regions is(are) formed directly on the shaftitself. Such transducers and related systems are described in, forexample, U.S. Pat. Nos. 6,260,423 and 6,047,605. In one form of suchso-called “collarless” transducers, the magnetoelastically active regionis polarized in a circumferential direction and itself possessessufficient magnetic anisotropy to return the magnetization in theregion, following the application of torque to the member, to the fullycircumferential direction when the applied torque is reduced to zero.Additional permutations of providing a polarized region have also beendescribed, such as in U.S. Pat. No. 8,438,937, which describes devicesused for detecting rates of change of torque in which the polarizedregion has a magnetic field applied either continuously or prior to ameasurement being obtained. Additional permutations of non-contactmagnetoelastic torque sensors have also been developed that providesignals indicative of the torque transmitted between radially separatedlocations of disk-shaped members, where one of more magnetized regionsradially located along the disk is used (see, e.g., U.S. Pat. Nos.8,424,393 and 9,046,430).

As described by U.S. Pat. No. 6,260,423, in constructions in which oneor more active, torque-sensing regions(s) is(are) formed directly on theshaft itself, the following basic conditions are required for the shaftand magnetic field sensors to function together as a torque measuringsystem: (i) the active region is ferromagnetic such that it can beremanently magnetized and is magnetostrictive (A); (ii) the activeregion is defined solely by the existence of remanent magnetization inthe circumferential direction; (iii) applied torsional stress causes thecircular remanence to develop an axial component, but does not alter theaxial component of magnetization within the non-magnetized regions ofthe shaft. Hence, there is a divergence of this component ofmagnetization and an external field thereby arises; (iv) magnetic fieldsor magnetic field gradients of sufficient amplitude developed in thesensing region from the application of torsional stress are large enoughto be measureable with a finite resolution of the magnetic field sensingdevice and associated acquisition system, and are substantially largerthan the usually encountered ambient field or magnetic field gradientsarising from parasitic sources; and (v) the transfer function of themeasureable magnetic field in the sensing region versus applied torqueor torsional stress to the shaft acting as the transducer is stable: (a)during repeated cyclic application of torsional stress; (b) with time;and (c) under any of the operational and environmental conditions thatthe shaft might be subjected to.

In such configurations, the torqued shaft or disk is desirably formed ofa polycrystalline material wherein at least 50% of the distribution oflocal magnetizations lies within a 90-degree quadrant symmetricallydisposed around the direction of magnetic polarization and has acoercivity sufficiently high that the field from the transducing regiondoes not create parasitic magnetic fields in regions proximate to theshaft of sufficient strength to destroy the usefulness, for torquesensing purposes, of the net magnetic field detected by the magneticfield sensor(s). For small stresses applied to the shaft, magnetizationwill change in part through domain wall motion; domain walls will movein such a way as to decrease the volume of domains magnetized at rightangles to the torsional stress axis, resulting in domain wall pinningand consequential hysteresis of magnetization acting in the direction inwhich torsional stress was last applied. Larger torsional stressesapplied to the shaft eliminate domain wall motion, but result in fieldsthat, if not sufficient to destroy the net magnetization, aresufficiently high to magnetize proximate regions of the shaft leading toremanent magnetization acting on the sensing region in the oppositedirection to that created by the torsional stress last applied.

As stated in U.S. Pat. No. 6,260,423, which again describes a“collarless” transducer, “hysteresis in the transfer function is theprimary source of imperfect performance.” The particular characteristicsof such magnetic hysteresis are dependent upon but not limited to: theshaft material characteristics; heat treatments applied to the shaft;geometry; operating temperature of the shaft; and torsional stress andhistory of the torsional stress applied. The accuracy and, ultimately,the usefulness of the torque measurement system is thus limited bymagnetic hysteresis as well as by the influence of temperature on thetorque transducer.

In practice, the selection of shaft material and the processes used tofabricate the shaft and its subsequent thermal and mechanical treatmentsare usually made to best fulfill the primary shaft function, i.e., themechanical transmission of torque, with little concern for whether thesefactors satisfy the preferences for magnetoelastic torque sensing. Evenin cases in which the material has characteristics that are ideal forboth the transmission of torque and torque sensing, there may bevariations or inconsistencies that cannot be controlled in practice thatinfluence the magnitude of magnetic hysteresis in the torque sensortransfer function. Examples of these variations are the shaft's chemicalcomposition, heat treatment, and stresses induced during fabrication,all of which can influence the characteristics of magnetic hysteresiswhen the shaft is used as a magnetoelastic torque transducer. As anexample of the variation allowed for in standard steels, consider acommon type of steel such as AMS 6265 (also known as AISI 9310). As perthe material specifications, the chemical composition of alloyingelements can range as follows: nickel, 3% to 3.5%; chromium, 1% to 1.4%;manganese, 0.45% to 0.65%; and silicon, 0.15% to 0.30%. As per thestandard heat treatment specifications for AISI 9310, the finishedhardness, which is often correlated with magnetic properties of thematerial (see, e.g., Belanger and Narayanan (2006)), is allowed to varyfrom 36 to 43 near the surface (Chandler (1994)).

Many efforts have been made to eliminate or reduce the effect ofhysteresis in magnetoelastic torque transducers with differentapproaches, including: improving materials and heat treatments thatmight offer ideal mechanical and magnetoelastic characteristics for thepurpose of both transmitting and measuring torque (see, e.g., Wun-Fogle,et al. (2009); Boley, Franklin, and Rigsbee (2000); Boley, Franklin, andOrris (2004)); processes for bonding and plating magnetoelasticmaterials to shafts (see, e.g., U.S. Pat. No. 7,401,531; Kilmartin(2003)); post-magnetization procedures to reduce hysteresis (see, e.g.,U.S. Pat. Nos. 7,308,835 and 7,350,425); and selecting the mosteffective excitation or other operation-related conditions (see, e.g.,Wakiwaka and Mitamura (2001)).

Other types of torque transducers, including contact type sensors, alsoexhibit hysteresis. One source of hysteresis present in torquetransducers based on a measurement of strain (which includes bothstrain-gauges and phase-shift torque-meters) originates from anon-linear relationship between the applied torque and angulardisplacement of the shaft, such as that caused by mechanical yielding(plastic deformation) of the shaft, which occurs when the appliedtorsional stress exceeds the proportional limit for the shaft material.Should the shaft plastically deform, upon relaxing the torsional stressto zero, the strain will not fully be recovered (Gere and Timoshenko,1997 pp.19 to 20 ). Should the strain-based torque sensor have anyaspect of plastic deformation or any non-linearity in the measurement ofstrain that is based on the history of prior torque applications, thetorque sensor will exhibit hysteresis. An additional source ofhysteresis in strain gauges that are bonded to a shaft stems fromslippage or movement in the polymer coating or bonding agent used to fixthe strain-sensing element to the object being measured (Enser, et al.(2017)). While the hysteresis error on these types of sensors is oftenvery small (e.g., <0.1% rated output) as compared to systems based oncircumferential remanent magnetization, in high accuracy torquemeasurement systems its contribution to net error cannot be neglected.

To date, given a transducer with less than ideal hysteresis, nouniversal approach exists to consistently reduce hysteresis to anacceptable level. This invention addresses, among other things, thislong-appreciated but still unresolved need.

Definitions

Before describing the instant invention in detail, several terms used inthe context of the present invention will be defined. In addition tothese terms, others are defined elsewhere in the specification, asnecessary. Unless otherwise expressly defined herein, terms of art usedin this specification will have their art-recognized meanings.

The terms “measure”, “measuring”, “measurement” and the like refer notonly to quantitative measurement of a particular variable, but also toqualitative and semi-quantitative measurements. Accordingly,“measurement” also includes detection, meaning that merely detecting achange, without quantification, constitutes measurement.

A “patentable” process, machine, or article of manufacture according tothe invention means that the subject matter satisfies all statutoryrequirements for patentability at the time the analysis is performed.For example, with regard to novelty, non-obviousness, or the like, iflater investigation reveals that one or more claims encompass one ormore embodiments that would negate novelty, non-obviousness, etc., theclaim(s), being limited by definition to “patentable” embodiments,specifically exclude the unpatentable embodiment(s). Also, the claimsappended hereto are to be interpreted both to provide the broadestreasonable scope, as well as to preserve their validity. Furthermore, ifone or more of the statutory requirements for patentability are amendedor if the standards change for assessing whether a particular statutoryrequirement for patentability is satisfied from the time thisapplication is filed or issues as a patent to a time the validity of oneor more of the appended claims is questioned, the claims are to beinterpreted in a way that (1) preserves their validity and (2) providesthe broadest reasonable interpretation under the circumstances. In thisregard, terms of approximation or degree (or relative terms) such as“about”, “substantially” and the like will be as understood by those ofordinary skill in the art, and will typically mean a range of valuesaround the stated value. If necessary in the particular context, such aterm will be understood to mean a range of values varying from by asmuch as 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10% more or 1, 2, 3, 4, 5, 6, 7,8, 9, or 10% less (or some smaller variance) than the stated value.

SUMMARY OF THE INVENTION

The object of the invention is to provide methods and systems forhysteresis compensation.

Thus, in one aspect, the invention concerns methods of hysteresiscompensation in a signal (e.g., an electronic signal) indicative of asensed-torque parameter experienced by a member, preferably atorque-transmitting shaft, upon application of a torque or atime-varying torque.

The methods of the invention include using a sense element disposed onor in sensing relation to a torque-transmitting member. Suchconfigurations allow for the generation of a signal indicative of asensed torque parameter from the torque-transmitting member uponapplication of a time-varying torque. The signal that is indicative ofthe sensed-torque parameter exhibits hysteresis error between about0.01% and about 20%. The signal is then computationally processed tocompensate for the hysteresis in the signal that is associated with theapplication of the time-varying torque. In preferred embodiments, suchprocessing utilizes stored information for the member that is correlatedwith a torque history or a degree of prior hysteresis compensation forat least one signal indicative of the sensed torque parameter upon priorapplication of a time-varying torque to the member in order tocompensate for magnetic hysteresis in the signal.

In some embodiments, the methods of the invention also involvecompensating for temperature-related magnetic hysteresis variation andtemperature-related changes in the transfer function of the magnetizedregion of the member.

Some preferred embodiments concern methods wherein the member features aremanently magnetized region or band, wherein the magnetizationpreferably has a circumferential orientation. The magnetized band may belocated on an inner or outer surface of a cylindrical shaft, or on thesurface of a disk (e.g., a flange, a gear, etc.) in which torque istransmitted between radially separated locations. The region may beoptionally transiently magnetized, in which the magnetization ispreferably refreshed (continuously, periodically at regular intervals,or intermittently). The sense element(s) is(are) disposed in sensingrelation to a magnetized band.

Some embodiments concern methods wherein the member uses a sense elementthat is disposed in sensing relation through direct contact on themember (e.g., a strain gauge bonded or otherwise operably affixed intorque sensing relation to the member) or where the sense element isfixed to the member and excited using a non-contact method (e.g., asurface acoustic wave-based method). Some embodiments concern methodswherein the sensed parameter is indicative of the angular displacementof the member under the application of torque (e.g., a phase-shifttorque-meter).

In some preferred embodiments, the sense element of the devices andsystems of the invention is disposed proximate to the member'smagnetized region in order to output a signal indicative of the torqueparameter when the member experiences or is subjected to a time-varyingtorque. In some embodiments, when the sense element senses a magneticparameter of the magnetized region, the magnetic parameter is optionallymagnetic flux or magnetization orientation.

In some preferred embodiments, the sense element is in contact with theshaft in, for example, the form of a strain gauge in which there is aresistive (or capacitive or other) change experienced by the straingauge indicative of a torque parameter when the member experiences or issubjected to a time-varying torque.

Some embodiments concern methods wherein the member transmitting torqueis a shaft or disk that is not fully continuous or solid over a 360° arc(a sector of a disk, a disk having a segment removed, atorque-transmitting arm or lever, etc.). In such embodiments, the senseelement(s) is(are) disposed in sensing relation on a portion of themember that is continuous and may optionally be fixed or be locateddirectly on the member.

In addition, the devices and systems of the invention preferably includea power supply and signal conditioning for the signal that areconfigured to output a signal indicative of one or more torqueparameters and signal conditioning to condition the signal to beaccepted by a processor operatively associated with the sense element,and configured to: (i) process signals output from the sense element todetermine the torque parameter(s); and (ii) compensate for hysteresis inthe signal. In preferred embodiments, the instant devices and systemsalso include a memory operatively associated with the processor that isconfigured to store data representing a torque history or degree ofprior hysteresis compensation for the magnetized region upon applicationof a time-varying torque to the member.

Other features and advantages of the invention will be apparent from thefollowing drawings, detailed description, and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B shows two graphs. The graph shown in FIG. 1A is atransfer function of measured magnetic field versus applied torque of18CrNiMo7-6 alloy steel with and without signal correction. The graphshown in FIG. 1B is a plot of the percentage of hysteresis (i.e., theratio of the maximum difference in the sensor output at any appliedtorque to the full-scale, span, or total range of torque applied, whiletorque is changed) for each transfer function as a function of appliedtorque.

FIGS. 2A and 2B shows two graphs. The graph shown in FIG. 2A is atransfer function of measured magnetic field versus applied torsionalstress for a system with hysteresis. A best-fit line is shown connectingthe minimum stress and maximum stress. The graph shown in FIG. 2B is aplot of the percentage of hysteresis for the transfer function shown inFIG. 2A.

FIG. 3 shows an example of torque peaks captured at a high speed ofsampling, as compared with nominal torque applied during a gearshiftevent in a motorsport application.

FIGS. 4A-4C shows three graphs. FIG. 4A shows three cyclic loads appliedas a function of time indicated by 1, 2, and 3. FIG. 4B shows thetransfer functions of measured magnetic field converted into voltsversus applied torsional stress from the three loading conditions ofFIG. 4A. FIG. 4C is a plot of the percentage of hysteresis for each ofthe three transfer functions of FIG. 4B as a function of applied torque.

FIG. 5 is a transfer function of measured magnetic field versus appliedtorsional stress for a system with hysteresis. The ascending anddescending limbs of the ±75 MPa transfer function are interrupted withtransfer functions defined by applied cyclical loads defined by −2MPa to35 MPa peak stresses.

FIGS. 6A and 6B shows two plots. FIG. 6A is a graphical illustration ofa typical “major” hysteresis loop of a ferromagnetic material. FIG. 6Bis a graphical illustration of a typical “minor” hysteresis loop of aferromagnetic material.

FIGS. 7A-7C shows three graphs. FIG. 7A shows the loading cycles appliedto a shaft as a function of time. FIG. 7B shows three transfer functionswith cyclic loads applied. Each transfer function has two cycles appliedas described by FIG. 7A. FIG. 7C is a plot of the percentage ofhysteresis for each transfer function as a function of applied torque.

FIGS. 8A and 8B shows two plots. FIG. 8A shows transfer functionsobtained with the shaft and sensor at 35° C., 60° C., 90° C., 120′C, and150° C. FIG. 8B is a plot of the percentage of hysteresis for eachtransfer function as a function of applied stress.

FIGS. 9A-9L shows 12 plots demonstrating a Preisach model for discreteinput values.

FIGS. 10A and 10B shows two plots. FIG. 10A is a plot of the percentageof hysteresis for each transfer function first presented in FIG. 7A.FIG. 10B is a plot of the percentage of hysteresis for each transferfunction after compensation is applied.

FIGS. 11A and 11B shows two plots. FIG. 11A is a plot of percentage ofhysteresis of the measured magnetic fields versus applied stress forapplied torsional stress cycles obtained at two temperatures, 35° C. and150° C., originally presented in FIG. 8. FIG. 11B is a plot of thepercentage of hysteresis for each transfer function after compensationis applied.

FIGS. 12A-12C shows a device capable of carrying out the accuracycorrection procedure in real-time, with units shown in millimeters. FIG.12A is a front view. FIG. 12B is a side view. FIG. 12C is an isometricview.

FIG. 13 shows a block diagram of a representative device according tothe invention that can implement hysteresis compensation in real-time.

FIG. 14 is an exemplary program flow diagram for implementing methodsaccording to the invention.

FIGS. 15A and 15B shows two isometric views. FIG. 15A represents a shaftmember transmitting torque, in which the shaft member features band(s)of remanent circumferential magnetization and proximate sense elementsto provide a signal indicative of torque. FIG. 15B represents the shaftmember from FIG. 15A in which a strain-sensitive element is used toprovide a signal indicative of torque.

FIG. 16 shows an isometric view representing a disk transmitting torque,in which the disk features band(s) of remanent magnetization andproximate sense elements to provide a signal indicative of torque.

FIGS. 17A-17B shows one isometric view (FIG. 17A) and one projectionview (FIG. 17B). FIG. 17A and FIG. 17B show the disk indicated in FIG.16 with features removed from the body of the disk such that it is notfully continuous about its diameter.

As those in the art will appreciate, the following detailed descriptiondescribes certain preferred embodiments of the invention in detail, andis thus only representative and does not depict the actual scope of theinvention. Before describing the present invention in detail, it isunderstood that the invention is not limited to the particular aspectsand embodiments described, as these may vary. It is also to beunderstood that the terminology used herein is for the purpose ofdescribing particular embodiments only, and is not intended to limit thescope of the invention defined by the appended claims.

DETAILED DESCRIPTION

1. Applications of Signal Correction Devices

The present invention describes devices and methods for increasing theaccuracy of a torque sensor system. This invention uses signalcorrecting devices and methods to acquire and digitize one or moreelectronic signals generated from torque sensing systems that exhibithysteresis, process such signal(s) to recognize the error from but notlimited to hysteresis, and then send a signal that is a function oftorque with the error factor(s) reduced or eliminated effectively inreal-time. The benefits of implementing such signal correction devicesand methods is that errors such as hysteresis that are inherent intorque transducers as described elsewhere herein, includingmagnetoelastic-based constructions using rings, collarlessconstructions, constructions using a coating, and sensor constructionsbased on a measurement of strain, can be reduced or eliminated, as canshaft-to-shaft variations in the characteristics of errors such ashysteresis. The capability of removing errors such as hysteresis from atorque sensor is significant for several reasons, including: 1) thetorque transducer can be manufactured from materials or usingheat-treatments that would otherwise be unsuitable for magnetoelastictorque sensors; 2) the accuracy of torque transducers can besignificantly improved, allowing accuracy requirements to be met forwhich they would otherwise be unsuitable; and 3) variability acrosstransducers can be reduced, allowing for greater consistency from onetransducer as compared with the next.

There are numerous applications in which measuring torque is ofinterest; however, traditional methods of measuring torque are notpractical due to limitations associated with the inability of suchsystems to fully address challenges posed by other factors such astemperature, speed, or packaging constraints, among others. While amagnetoelastic torque sensor may be able to overcome these issues, it isoften the case that the shaft transmitting torque is manufactured from amaterial that when used as a transducer would have inaccuraciesassociated with hysteresis and temperature that exceed the accuracyrequirements for the application. This is particularly true for theenergy and military sectors, in which machinery transmitting torqueoften operates at high temperature (e.g., >100° C.) and at highrotational speeds (e.g., >14,000 rpm). Considering the energy sector inparticular, standard steels used to manufacture wind turbine shafts are18CrNiMo7-6 and AISI 4340. Testing these materials for use as amagnetoelastic transducer has shown that they display typically 4% ormore error due to hysteresis, as well as temperature-dependence in thetorque sensor transfer function typically of the same magnitude (>4%).Errors of this magnitude are well beyond what is considered to beacceptable for a torque sensor system. FIG. 1A is a plot of the outputof a magnetoelastic torque sensor as a function of applied stress on ashaft manufactured from 18CrNiMo7-6. The uncorrected output is shown inthe dashed trace, which has approximately 4.5% error from hysteresis.Hysteresis as a function of applied torque is shown in FIG. 1(b). Thisinvention allows for the removal or significant reduction of this error,as shown by the solid trace, as the error from hysteresis is now lessthan about 0.5%, which would generally be acceptable for use in thismarket. As such, this invention makes it possible to use materials fortorque sensing that were previously unsuitable for such applications.

The need to use pre-existing or widely available materials isparticularly true for the military sector where, due to the high cost ofcertifying components, there are many potential applications for atorque sensor in which the existing shaft (or shafts made of the samematerials as those already certified) is required to be used as thetransducer; however, the shaft is manufactured from a material that whenused as a transducer will not be accurate enough to meet therequirements for the application. As an example, Kari, et al. (2012)documented the configuration and accuracy of a torque-meter that wasretrofitted to the United States Navy LCAC hovercraft, which used theexisting engine output shaft manufactured from AMS 6265 as thetransducer shaft. While the accuracy of the torque-meter was 2.5% whenspecified across a wide-range of temperatures, it was only brought tothe required 2% by calibrating the sensor at the specific operatingtemperature of the engine, a practice that is often not an option inother installations. Reducing or eliminating errors associated withhysteresis and temperature would make it possible to utilizemagnetoelastic torque sensor technology in many applications for whichit is currently unsuited in terms of accuracy, reproducibility, etc.

A similar situation exists for markets such as the laboratory torquesensor market, in which accuracy requirements are typically 0.25% orbetter, that cannot be easily met with conventional magnetoelastictorque sensor technology. Even with other torque measurementtechnologies based on the measurement of strain (e.g., strain gauges andphase-shift torque sensors), hysteresis contributes to the net error ofthe torque transducer and thus limits the ultimate accuracy. However, byimplementing real-time signal correction to reduce errors fromhysteresis and, in preferred embodiments, temperature, accuracy andother requirements can be met. These examples demonstrate how theapplication of signal correction in real-time makes it possible tosignificantly expand the range of applications and markets to whichtorque sensing technology is suited.

2. Influence and Mechanisms of Hysteresis

Hysteresis in a torque measurement system may not be an issue inspecific scenarios and applications. For systems in which the loadingprofile is significantly asymmetrical, such that decelerating torquesapplied to the shaft are negligible relative to accelerating torques andonly one nominal load is applied and is of interest, such as a maximumpower condition, the presence of hysteresis may be inconsequential.Consider FIG. 2A, and points “A” and “B” in particular. If the loadprofile applied to the shaft only traverses from “A” to “B” and back to“A”, and only the measurement at “B” is of interest, the hysteresisbetween “A” and “B”, represented in FIG. 2(b) as a percentage of theapplied torsional stress may be inconsequential, as even significanthysteresis error may not be a factor with regard to the usefulness ofthe sensor. The presence of hysteresis may also be inconsequentialshould the primary function be that of recognizing the presence (or lackthereof) of specific frequency components, such as the measurement oftorsional vibration or periodic impulses created by reciprocating eventsthat are a function of the operating state of the system, which mayinclude (without limitation), for example, gear mesh harmonics in atransmission, phase actuation in an electric motor, or the firing ofpistons in an internal combustion engine.

While these events may potentially be quantified by torque measurement,should the frequency of interest be high (e.g., greater than about 5kHz) and the amplitude of oscillation of interest be small as comparedwith the nominal torque or resolution of the measuring system, U.S. Pat.No. 8,438,937 describes methods and devices to measure these signaturesby directly measuring the rate of change of torque on atorque-transmitting member acting as a magnetoelastic transducer inwhich the transducer may demonstrate hysteresis. Should a precisemeasurement of the peak-to-peak magnitude of oscillating torques viatorque or rate-of-change-of-torque be required, hysteresis error may beproblematic; in particular, as is shown below, hysteresis error oftenmanifests itself as a change in gain, influencing smaller amplitudeexcursions especially.

Often in practice there are both accelerating and decelerating torquesapplied to a shaft on which torque is being measured, as well as“overload” torques applied transiently that greatly exceed the nominalor steady-state measurement range of interest. For example, it may be ofinterest to measure the nominal torque produced by an engine, butimpulsive mechanical events such as gearshifts or driveline resonancesmay produce transient torques of relatively short duration that havepeak magnitudes of several times the amplitude of the nominal torque. Inthese conditions, the inaccuracy induced from magnetic hysteresis maygreatly limit the usefulness of the sensor. An example of the peaktorque induced during a gearshift in a motorsport application is shownin FIG. 3, in which the peak torque is twice the magnitude of thenominal torque applied to the shaft.

The influence of hysteresis on the output of a torque sensor system maynot always be apparent. For a torque sensor system designed for aparticular maximum rated capacity, torque cycles applied from themaximum extremum to the minimum extremum and again back to the maximumextremum define the “major” loop. Applied torque cycles that have peakmagnitudes that are within the rated capacity are described as “minor”loops. For situations in which the overload range defines the majorloop, but the measurement range of interest is defined by a minor loop,errors associated with the minor loop as compared with the actual torqueapplied during the minor loop can be described as either gain or offseterrors (Fraden (2010)).

FIG. 4 consists of three plots demonstrating a situation in whichhysteresis is observed to cause a gain error in minor loops that is afunction of the peak magnitude defining the minor loop. FIG. 4A is aplot of the applied torsional shear stress as a function of time, inwhich a major loop consisting of two torque cycles is applied from −14MPa to 150 MPa indicated by “1”, followed by two minor loops: from −11MPa to 108 MPa indicated by “2”, and from −11 MPa to 50 MPa indicated by“3”. FIG. 4B shows the transfer function of the output signal of thetorque sensor system as a function of the applied torsional stress foreach stress range. FIG. 4C is a plot of hysteresis as a percentage ofthe full-scale range defining each respective loop, in which it can benoted that the hysteresis for the major loop has a maximum value of3.5%. In this example, the offset (output signal at zero torque input)at the unloaded condition (0 MPa) is quite consistent between the threeloading conditions, but if a best-fit line is used to compare thetransfer function for each loading profile, the effective difference ingain (slope) between the 150 MPa major loop and 105 MPa minor loop is0.75%, while the difference in gain between the 150 MPa major loop andthe 50 MPa minor loop is 3%. The significance of this is that should acalibration be applied to the minor loop (−10 MPa to 50 MPa) that iscreated from a regression of the full-scale range (−20 MPa to 150 MPa),when operating in the range of the minor loop, a 3% error in gain is tobe expected.

The variation in offset as caused by hysteresis is most obvious whencomparing equivalent minor loops following significant applied loads ofopposite polarities. FIG. 5 is a plot of a +/−75 MPa major loop with 1%hysteresis indicated by “1”, scaled to the range of 2.5 MPa to 37 MPa.The same cyclic minor loop of range −2 MPa to 35 MPa indicated by “2”and “3” was applied while traversing ascending and descending limbs ofthe major loop. The slope (or gain) of each minor loop is equal andapproximately 1.5% less than that of the major loop. The offset error ofthe minor loops as compared with the major loop is a reasonable 0.7%;however, as compared with the span of the minor loops, the offset errorbetween the minor loops is nearly 3%. The significance of this is thatshould the torque sensor system be subjected to an overload ratio(defined as the ratio between maximum transient torque and nominalsteady-state torque of interest) of only 2:1, even should the major loopdefining the overload range have only 1% hysteresis, the minor loopdefining the measurement range of interest would have an error thatexceeds 3% as a function of its span. In some applications, inparticular when torsional resonances are excited during the typicaloperation of the machine, overloads may be applied that are well beyondthe 2:1 ratio, further exacerbating this type of potential error andfurther highlighting the usefulness of the invention. Examples of suchapplications include but are not limited to: high-speed, low inertiavariable frequency drive electric motors and internal combustionengines.

Hysteresis as observed in torque sensors can act in either the samepolarity as that in which the applied torsional stress develops amagnetization in the sensing region, or in the opposite polarity. It isuseful to consider a standard BH curve for a ferromagnetic material asshown in FIG. 6 given the fact that magnetic fields arise from theactive circumferentially magnetized region of the shaft and these fieldspervade not only the space in the sensing region where the fieldsensor(s) is located but also the space occupied by the shaft itself,which can be described as non-active but magnetize-able proximatematerial. Ferromagnetic materials are readily characterized by themagnitude of the magnetization changes induced by magnetic fields. Sincethese characteristics are not single valued functions, they areconveniently described by a plot of magnetization M versus field H, as His cyclically varied over a symmetrical bipolar range. The salientfeatures of such a “major” hysteresis loop are indicated in FIG. 6A,wherein the limiting fields are sufficient for the magnetization to showsigns of approaching saturation. The “minor” loop is just as relevant,such as that shown in FIG. 6B. Even for small excursions of an appliedfield, the resulting magnetization alterations are seen to include someirreversibility, or finite remanence and coercivity. This is relevant tothe sensor as non-active regions will be exposed to some field from theactive regions, and by virtue of the finite remanence and coercivity ofthe material comprising these regions, the resulting magnetization inthe non-active regions will vary in a hysteretic fashion with appliedtorque. As a result, these previously inactive, newly magnetized regionsthemselves contribute field components in the sensing region and aroundthe shaft. Furthermore, for excursions of magnetization in the activeregion based on applied torsional stress to the shaft, non-activeproximate material can become remanently magnetized. Upon relaxingtorque and thus the magnetization to an unloaded condition,magnetization will be present in the opposite direction to that createdby the applied torque. Applying symmetrical loading cycles in whichnon-active regions become remanently magnetized, in which the magneticfield emitted from the remanently magnetized non-active regions is ameasureable portion of the total magnetic field in the sensing region,will result in a hysteresis loop having a clockwise orientation (CW)often designated as hysteresis having a negative polarity. See alsoGarshelis and Cuseo (2009).

As described in U.S. Pat. No. 6,260,423, which again describes a“collarless” transducer, as the coercivity of standard plain carbon andlow alloy steels are typically in the range of 5 to 50 Oe, and as thecoercivity required to rotate the magnetization through vector rotationis beyond 500 Oe, the principal process by which magnetization isaltered in these materials is not vector rotation but, rather, domainwall motion that is subject to domain wall pinning. While not wishing tobe bound to a particular theory, should domain-wall motion beresponsible for the change in magnetization (based on domain wallpinning), upon relaxing torsional stress and thus the magnetization toan unloaded condition, some magnetization will remain present acting inthe same direction as that created by the originally applied torque.Should sufficient torsional stress be applied, domain wall motion willbe eliminated as a way to change magnetization. Applying loading cyclesto a shaft within a range of torsional stresses that exhibit domain wallpinning will result in a hysteresis loop having a counter-clockwiseorientation (CCW) often designated as hysteresis having a positivepolarity in which the maximum magnitude of the hysteresis is dependentupon the magnetization and material characteristics.

These two described mechanisms of hysteresis responsible for negativeand positive hysteresis, respectively, are evidenced by specific loadingprofiles that can be applied to selectively produce transfer functionswith: positive hysteresis, negative hysteresis, or minimal hysteresis bybalancing the positive and negative hysteresis. An example of this isshown in FIG. 7, in which FIG. 7A is a plot of the applied torsionalstress as a function of time, in which a major loop consisting of twotorque cycles is applied from 300 MPa to −300 MPa indicated by “1”. Thisis followed by three minor loops consisting of two torque cycles each of210 MPa to −210 MPa indicated by “2”, 180 MPa to −180 MPa indicated by“3”, and then 85 MPa to −85 MPa indicated by “4”. FIG. 7B shows thetransfer function of the output signal of the torque sensor system as afunction of the applied torsional stress. FIG. 7C is a plot ofhysteresis as a percentage of the full-scale range defining the loop. Itcan be noted that the hysteresis is of a positive polarity for loopsdefined by lower peak magnitudes of applied torsional stress indicatedby “3” and “4”, whereas as the peak magnitude of applied torsionalstress increases, the percentage of hysteresis decreases, ultimatelyswitching from positive polarity to negative polarity.

3. Measurement Error is Repeatable and Deterministic

This invention recognizes that the hysteretic component in the measuredmagnetic field as well as the temperature dependent changes in thetransfer function (of the measured magnetic field versus appliedtorsional stress to the transducer) for magnetoelastic-based torquesensors and hysteresis in torque sensors based on other constructions isrepeatable and deterministic. If the measured hysteresis ischaracterized over a finite number of applied torsional stress cycles tothe transducer shaft, which characterization can be described as majorand minor loops, as well as characterized for conditions that influencethe hysteresis, such as temperature, should a new torsional stress beapplied to the shaft and the temperature considered, the magnitude andpolarity of hysteresis can be predicted and thus compensated for.Another factor to consider, while negligible for certain particularmagnetic field and strain measuring devices, should the magnetic fieldmeasuring device induce its own hysteresis on the measurement ofmagnetic field, is the fact that it, too, would contribute to thehysteretic component in the measured magnetic field, which can also befound to be repeatable and deterministic.

In order to be deterministic, the hysteretic component and temperaturedependent changes need to be repeatable. With respect to the applicationof quasi-static torsional stress cycles under the same conditions, inpractice the output has been shown to be repeatable with applied cycles.Examples of this can be seen in FIGS. 1, 4, 5, and 7, as each plot showstwo cycles applied, in which the hysteresis is observed to be nearlyidentical during each applied cycle. Applying additional cycles does notinfluence the hysteresis. On the other hand, a difference in thecharacteristics of hysteresis will be observed if torsional stresscycles are applied at a different temperature. An example of theinfluence of temperature upon hysteresis is shown in FIG. 8, in whichthe measured magnetic field is plotted as a function of the sametorsional stress cycles for increasing temperatures. As shown in FIG.8B, the polarity of hysteresis is positive at 35° C. but becomesincreasingly negative as temperature increases. The influence oftemperature on hysteresis can also be shown to be repeatable, such thatthe same hysteretic component would be observed if the same history oftorsional stress and temperature were to be applied.

Although not found in typical situations, there may be circumstances,such as when requiring extremely high accuracy from the torquetransducer, in which hysteresis may be found to have a time-relatedcomponent; however, it may also be found to be repeatable anddeterministic. A time-related component of hysteresis may not beunexpected, based on the following phenomena in magnetic materials: (i)dis-accommodation, which is a time dependence of permeability associatedwith the diffusion on ions through the ferrite lattice of the transducermaterial; (ii) settling time and relaxation associated with the durationthe load is applied; and (iii) eddy currents that act to limit themagnitude of rapidly changing magnetic fields.

There are additional considerations that may also be taken into accountthat may not easily be quantified. Examples include the presence oflocal magnetic fields, local permeable material in which thepermeability may be dependent on a parameter not being measured andquantified, and compressive or tensile stresses applied to the shaftthat may be seen to influence the characteristics of hysteresis.

4. Models of Hysteresis

As has been described, the hysteretic component of the measured magneticfield is repeatable and deterministic, but in order to use this forsignal correction, a model of hysteresis is required that can be used tocalculate the expected hysteretic component using the measured signalfrom the torque transducer as at least one of its inputs. There is nopaucity of technical papers describing models that have been developedto model hysteresis. While other approaches have been described (see,e.g., Dupre, et al., (2001)), most modeling efforts either can becategorized as differential equation models or continuous operatormodels that are a function of history. Examples of differential equationmodels are: the Bouc-Wen model (Ikhouane and Rodellar (2007)); the Duhemmodel (Visintin (1994)); and the Jiles-Atherton model (Jiles andAtherton (1986)). The Preisach continuous operator model was firstsuggested in 1935 (Preisach (1935)), and is designed to model ahysteresis loop as nonlinear relay operators called “hysterons”uniformly distributed on a Preisach α, βplane, with a Preisach triangledefined and bounded by the α, βline. Each hysteron is modeled as a relaywith its on/off state defined by specific thresholds with weightsassigned according to its state, in which the weighting is often definedusing a density function.

While all models use different approaches and methods, all methods startwith measuring and storing the variation in the system output as aninput increases from a specified minimum to a specified maximum and thenfrom maximum to minimum values. Hysteresis is manifested by thenon-coincidence of the ascending and descending data sets. When thegathered data is plotted, the lines connecting the data pointsrespectively comprise ascending and descending limbs of a majorhysteresis loop. Similar measurement and data storage of first ordertransitions (e.g., changes in direction as the input was increasing fromits specified minimum before the maximum is reached, and changes in thedirection as the input was decreased from its maximum before the minimumis reached) are also typically performed. Similar measurements and datastorage may also be made on second order transitions (when the directionof input variation following a first order transition is again reversedbefore the input reaches the extreme value in its new direction).Plotted data following such transitions describe “minor loops”. Fromthis experimental data, parameters are estimated to create a suitablyaccurate model of the complete input versus system output transferfunction.

5. Application to Torque Transducer Signal Correction

For torque sensor systems that operate across typical limits of appliedtorsional stress (e.g., less than 150 MPa applied torsional stress) thatuse materials such as AMS 6265 or AMS 6419 and operate in thetemperature range of 20° C. to 80° C., experimental data has shown thehysteretic component of the measured magnetic field is well described bya Preisach model with a uniform hysteron density, which can beimplemented without computationally intensive algorithms. Within theselimits, there has also not been an indication that there are significantdeviations from the model based on non-ideal conditions such as (i)reptation, which would be observed as hysteresis being dependent on thenumber of loading cycles applied, (ii) saturation, (iii) asymmetry,which would be observed as hysteresis being dependent on the polarity ofthe load applied, or (iv) time influences, such as a dependence on therate at which the torque is applied to the transducer shaft. Given thegood fit provided by the Preisach model, other predictive models,including the previously referenced models (e.g., Jiles-Atherton), canalso be applied to model the hysteretic component of the measuredmagnetic field responding to an applied torsional stress. It is alsoworth noting that the input into the model need not be limited to themeasured magnetic field, and can include other parameters that arecorrelated or associated with applied torque and the hystereticcomponent such as, for example: (i) measurements of displacement; (ii) ameasurement of time, such as in the case of reciprocating systems inwhich time can be related to the applied torque; or (iii) externalevents such as the opening of a valve that may result in awell-quantified change in applied torque. As the model of hysteresiscomputes the magnitude and polarity of the hysteretic componentdependent on the input of applied torsional stress (or magnetic field,or other parameter that correlated with applied torque, such as ameasured displacement, element of time in a reciprocating system, etc.)should the hysteresis be relatively small (for example, less than about5%), the uncompensated sensor output is able to provide the inputdirectly into the model. In such cases, the computational result of theapplied model of hysteresis can be inverted in polarity and be directlysuperimposed on the measured magnetic field. If the hysteresis issignificantly large such that the measurement of the applied torsionalstress is significantly flawed, or the non-ideal conditions mentionedabove are seen to be significant factors, a more numerically involvedmodel (such as, for example, a Preisach model with non-uniform hysterondensity or a rate-dependent model such as the Bouc-Wen model) ispreferably employed for compensation.

As described by the Preisach model, the active area of the Preisachtriangle is defined as that represented by the limits between theminimal extrema or the value of alpha at the previousnegative-to-positive transition and current α. Area representing historyis held prior to each negative-to-positive transition. Should α descendbelow a previous negative-to-positive transition, or ascend such thatthe current β is greater than that of a previous positive-to-negativetransition, the Preisach model uses a “wiping out” function such thatthis previously generated area is either eliminated or absorbed. Whenconsidering the active area of the Preisach model, when ascending thehysteresis loop (increasing value as compared with the previous point)the area of integration is a triangle bounded by: (i) the α, βline; (ii)a is bound by the previous negative-to-positive transition (or minimalextrema) and current value α; and (iii) β is bound by the previoustransition and current value of β.

When descending the hysteresis loop, the area of integration is aparallelogram bounded by: (i) the α, β line; (ii) a is bound by theprevious negative-to-positive transition and the current value of α; and(iii) β is bound by the positive-to-negative turning point.

An example of the Preisach algorithm is shown in FIG. 9, in which fiveinputs (α) are considered: α₁=0; α₂=2000; α₃=1200; α₄=1800; and α₅=0, inwhich these units can represent any digital quantity (e.g., torque,bits, Volts, etc.). Although five points are considered, just as torquechanges continuously but is sampled in small but finite steps (dependentupon the acquisition rate of the signal), the values between thesepoints are also considered, calculated in steps of 2 units in thisexample (e.g., 0→42000 is calculated at 0, 2, 4, 6, . . . 2000).

FIG. 9 consists of 12 plots (FIGS. 9A-9L), in which each columnrepresents the current value when moving from α to α_(i)+1 (e.g., plotsA, E, I represent α₁=0 to α₂=2000). The first row (FIGS. 9A, B, C, andD) is a plot of the α, β line and area represented on the Preisach planefor the current value of α. The second row (FIGS. 9E, F, G, and H)represents the area computed divided by the full-scale value of α. Thethird row (FIGS. 9I, J, K, and L) rotates the normalized area such thatit is fitted about the abscissa. As shown in FIG. 9A, moving from aninput of 0 to 2000, the area is integrated bound by the α, β line withlimits of integration defined by α and β, with the resulting area beingrepresented as a triangle. FIG. 9 demonstrates that the hysteresis atα=0 is the same as at α=2000, such that the ascending limb of the majorloop has been traversed. As shown in FIG. 9B, moving from an input of2000 to 1200, the area is effectively removed bound by the α, β linewith limits of integration defined by α and β leaving the parallelogramas shown. As shown in FIG. 9C, moving from an input of 1200 to 1800,area is again added; however, the parallelogram from FIG. 9B remainsacting as a “memory” of the previous history of inputs. The new area isadded as a triangle defined by the turning point at 1200 and limits ofintegration defined by α and β. FIG. 9C demonstrates the hysteresis isagain approaching 0.

As shown in FIG. 9D, moving from an input of 1800 to 0, all area iswiped away, thus completing the descending limb of the major loop.

It is clear from FIG. 91 that the output of a descending segment isclearly different from that of an ascending leg of hysteresis. Forconditions in which a uniformly weighted hysteron density function is anappropriate fit and the error between the applied torque and themeasured torque is small (e.g., less than 5% deviation), the Preisachmodel as described only requires two parameters to be applied to scaleit to an experimentally determined hysteresis curve: (i) the range ofthe input signal and thus the major loop, which sets the full-scalevalue of a; and (ii) the magnitude of hysteresis present in the majorloop as a function of the span of α, which is used to set the densityfunction in the traditional model. Alternatively, the magnitude ofhysteresis can be used to adjust the slope of the α, β line when using auniform density function, resulting in the computational demands ofrunning the model to be significantly decreased as the density functionneed not be computed on an element-by-element basis; instead, simpletrigonometry can be used to solve for the total area present. The thirdrow of FIG. 9 (FIGS. 9I-L) can be inverted and superimposed directlyonto the signal with hysteresis, thus removing hysteresis from thesignal. An example of the described method being applied to compensatefor hysteresis is shown in FIG. 1.

6. Addressing Departures from Standard Models

While hysteresis in a torque sensor system can often be well-describedusing a specific, uniformly weighted distribution, as the torsionalstress applied to the shaft or temperature is significantly increased,the hysteresis observed in the torque sensor as a function of thehistory of torsional stress changes significantly, such that applyingthe originally applicable weighted distribution will result in at best,a less than ideal fit of the actual hysteresis, but at worst will yieldsignificant inaccuracies between the model and the actual data. While atraditional non-uniform density function can be applied to fit dataobtained at higher temperatures and torsional stresses, depending uponthe processing power available, it may prove more practical to continueto use a uniform density function in which the slope of the α, β line isused to control the magnitude of hysteresis being compensated for, but:(A) bounds are placed on the input range of the hysteresis compensationalgorithm. This is equivalent to using a non-uniform density function,in which the density is set to 0 after a specific input; (B) a secondmodel dedicated to events that the primary model cannot easily describeis employed; and (C) the slope of the α, β line is adjusted as afunction of temperature or other environmental variables, rather thanmodify the weighting of the density function. The output area for agiven input will be proportional to the slope of the α, β line, which isequivalent to changing the average uniform density when applying adensity function.

With respect to the influence of increasing magnitude of torsionalstress, consider FIG. 7, in which hysteresis is positive for a minorloop, such as those indicated by “3” and “4”, but becomes increasinglynegative as the applied torsional stress is increased on subsequentcycles indicated by “1” and “2”. To handle such conditions, (A) and (B)(above) can be employed. As it is observed that positive hysteresis isonly present for a limited range of torsional stress, the input into thehysteresis compensation model can simply be bound to a finite range. Asdescribed in (A), this is equivalent to using a non-uniform densityfunction, in which the density is set to 0 after a specific magnitude.As negative hysteresis develops only after the input exceeds a certainrange, a secondary model can be used that is as straight-forward assuperimposing a finite value onto the output of the primary model, inwhich the secondary model's finite value is a function of the maximumexcursion of torsional stress and temperature. The magnitude of thissecondary model's output is only relaxed after the polarity of the inputis reversed. These two methods are applied to the data first shown inFIG. 7. FIG. 7A is copied into FIG. 10A, in which the results ofapplying compensation to this data are shown in FIG. 10B.

As for the influence of increasing temperatures, consider FIG. 8, whichdemonstrates that the polarity of hysteresis is initially positive butbecomes increasingly negative for increasing temperatures. To handlesuch changing conditions, both (B) and (C) (above) can be employed. Asthe magnitude of hysteresis is decreasing as temperature increases, theslope of the α, β line can be decreased as a function of temperature.The same secondary model as in the previous example can also beemployed, but the threshold dictating when the secondary model isapplied can be adjusted as a function of temperature. As shown in FIG.11A, the two methods are applied to the data first shown in FIG. 8, butonly consider the temperatures of 35° C. and 150° C. The compensatedoutput is shown in FIG. 1B; the influence of hysteresis that is afunction of temperature is reduced from about 0.5% magnitude to lessthan about 0.2% magnitude.

7. Requirements for Real Time Signal Correction

While hysteresis models and compensating methods ofsimulated/experimental data have been described, there are severalpresumptions in the acceptance of the output signal of a systemutilizing signal correction as an improved indicator of the actualtorque, due to the input into the signal correction model being a validmeasurement. These include: (i) the bandwidth of the torque sensingsystem must be sufficient to measure the actual applied torqueamplitude, when applied at any rate. For particular shaft materials orconstructions of the sensor, eddy current and time effects need to benegligible. For example, enclosing the sensor or shaft in a conductivematerial such as brass is likely to lead to a significant time effect orattenuation of the measured magnetic field that is a function offrequency; (ii) the measurement of the magnetic field or input into themodel needs to be nominally free of electrical noise or spurious inputs,as these appear as inputs into the model that will result in a responsefrom the output of the signal correction algorithm; (iii) the systemmust be capable of performing the hysteresis compensation algorithmquickly enough that any torque excursion(s) is (are) quantified andprocessed regardless of how quickly it is (they are) applied; and (iv)compensation of hysteresis requires continuous acquisition of the sensedquantity. If, for example, a torque is applied, relaxed, or otherwisechanged when the sensor and/or compensating electronics are unpowered,the compensator will no longer have the knowledge (information) requiredto calculate and thus compensate for the hysteresis. Unless (i) theshaft can be brought to one or the other torque extrema to allow thecompensator to “reset” itself, (ii) the applied inputs are consistentand repeatable such that default values can be set that are reflectiveof the operating conditions, (iii) the system will never be powereddown, and/or (iv) non-volatile memory is always used, it is necessary tostore either the operating state or a history of inputs that allow theoperating state to be recreated in non-volatile memory, thereby allowingthe current state to be restored upon power-cycling. Otherwise, uponrestarting, the inputs into the signal correction algorithm and itssubsequent output will not match what is required for properidentification of the state of the torque transducer.

8. Representative Embodiments

FIG. 12 is a mechanical drawing detailing the dimensions and isometricview of a device according to the invention that contains the magneticfield measurement drivers, signal conditioning, microcontroller, andassociated hardware as shown in the block diagram in FIG. 13 required tocarry out the signal correction procedure in real-time. Real-time signalcorrection requires a number of stages of hardware, represented in FIG.13. The first stage of hardware is the sensing technology indicated by10, such as a magnetic vector detection device or strain measuringdevice with associated driver circuitry indicated by 12, in which themeasured quantity is a function of torque but also has a component ofhysteresis. Optionally, the magnetic field measurement driver may beconnected to a microcontroller to carry out a number of tasks, such asproviding or influencing clock signals used by the drivers used for thesensing technology. The magnetic field measurement technology must havea bandwidth capable of measuring any significant frequency componentspresent that are indicative of torque without attenuating or amplifyingtheir magnitude. In practice, torsional oscillations and thus the signalmay contain frequency components beyond several kilo-Hertz, such thatthe bandwidth is required to be at least this frequency or higher. Theoutput of the sensing technology is often in an analog voltage orcurrent format, although might also be represented by a frequency orphase, or be directly converted into a digital format.

In particular, when the measured signal is an analog format, signalconditioning electronics 13 are preferably included to properlycondition this measured electrical signal for successful conversion intoa digital format using an analog to digital converter (ADC) 15. An ADCand signal conditioning may also be used to convert the analog output ofother transducers such as temperature sensors 11 into a digital format.The signal conditioning may include but is not limited to adjustments tothe span and offset of the signal, as well as applying filtering toremove any frequency components above half the intended ADC samplingrate, as these frequency components would otherwise be aliased. Thesampling rate used should be at a minimum of twice the rate of thehighest frequency components that are expected to be present (as statedabove, often beyond several kilo-Hertz); however, a factor of 10 or morewill be ideal as it will allow the peak magnitude of higher frequencycomponents to be precisely quantified. The digital signal should then beavailable to a computational device such as a microprocessor or DigitalSignal Processor (DSP) indicated by 16, capable of carrying out thecompensation algorithm, with either internal or external non-volatilememory indicated by 17, and optional volatile memory to execute thealgorithm on, in which the digital signal represents the appliedquantity of torque with a component of hysteresis.

This signal can then be passed into a hysteresis compensation algorithmeither point-by-point or by providing multiple values to be processedsimultaneously. A flow diagram of firmware capable of carrying out thecompensation algorithm is shown in FIG. 14. Required by the signalcorrection algorithm are a number of parameters that are a function ofthe input torque signal and temperature. These may include, but are notlimited to, (i) a density function, (ii) the range over which thepositive hysteresis is dominant, (iii) the percentage of hysteresischaracteristic of this range, (iv) the range wherein negative hysteresisis observed which may be described as remanent hysteresis, and (v) ascaling factor to define how remanent hysteresis is a function ofapplied torque. Additional parameters can also be used to providecompensation for effects such as time effects (e.g., eddy currents,influences of reptation or a dependence of hysteresis on the number ofcycles applied, etc.).

With respect to obtaining a parameter, the torque sensor transducer tobe optimized is preferably subjected to full-scale (or rated capacity)torque cycles followed by several minor loops, often at 75%, 50%, and25% of the magnitude of the full-scale torque cycle. To account for theinfluence of temperature on the output of the sensor, the same torquecycles are applied when the shaft and matching sensor are at a differentoperating temperature. Typically the shaft and sensor are heated to themaximum operating temperature of the sensor, and torque transferfunctions are obtained at a variety of different temperatures throughoutthe test. This data is used to obtain the optimization parameters forthe torque transducer with respect to sensitivity and offset of theshaft as a function of applied torque, as well as the various hysteresiscompensation parameters that are a function of temperature.

These parameters can be stored in firmware, the non-volatile memory ofthe hardware, or sent dynamically to the microcontroller through aninterface such as CAN (Controller Area Network), such that they can beutilized by the compensation algorithms. Turning to FIG. 14, afterpowering-on the microcontroller 20, if these parameters are stored onnon-volatile memory, they can be read and loaded by 21 and 22. A historyof previously applied torques, values of hysteresis compensation, orother related parameters that can be used to reinitialize the hysteresiscompensation algorithm 23 can also be read and loaded.

After initializing the microcontroller, timer-based interrupts arepreferably used to periodically carry out events associated with theoperation of the firmware at a set rate, with operations such asupdating temperatures operating relatively slowly such as at a rate of 1Hz, and the analog to digital sampling of the magnetic field andassociated compensations happening relatively quickly (e.g., 20 kHz)allowing rapidly changing torque transients to be captured and processedby the algorithms.

Parameters associated with hysteresis compensation can be changeddynamically based on external inputs or environmental parameters such astemperature (24, 25). Temperature is typically obtained using atemperature sensor built into the magnetic field sensor assembly placedproximate to the torque-transmitting member. The magnetic field iscontinuously sampled by an ADC 26, and this sampled signal can havefurther filtering and processing such as temperature compensationapplied (27, 28, 29). If the percentage of hysteresis is relativelysmall (<5%), the hysteretic component can be calculated using theparameters for the model (30), in which the output can be inverted andsuperimposed onto the measured quantity of torque (31). Alternatively,the measured torque signal can be modified directly. Both cases resultin a processed signal in which the measured components of hysteretic andtemperature dependent error are removed or diminished. If the hysteresisis relatively large, such that the input to the model as measured by thesensor is not a reasonable estimate, a more advanced or iterative modelcan be implemented, ultimately also resulting in a processed signal withthe hysteretic and temperature dependent error removed or diminished.This processed signal can be manipulated further, by applying scaling,filters, signal processing techniques and tools, or normalizing tospecific ranges (32). The further processed signal is then sent to anoutput buffer (33). Typical transmission methods for this processedsignal include but are not limited to using a digital format such asController Area Network (CAN; 35), a frequency representing the signal,or restoring this digital signal to an analog voltage or currentcomponent (34). The CAN and analog transmission methods are shown inFIGS. 13 (18 and 19, respectively).

FIGS. 15A and B are mechanical drawings representing a shaft sectionwith centerline 50 with diameters indicated as 51 and 52, respectively,in which the inner diameter is optional. Applied torque is indicated byoppositely oriented arrows 56 and 57, which produces a shear stress,strain, and subsequent angular twist in the shaft. In a preferredembodiment as represented by FIG. 15A, the shaft is magnetized to haveone or more bands of remanent magnetization 54 (in alternativeembodiments, magnetized rings or hoops could be attached to the shaft).One or more sense elements 55 located proximate to the shaft provide asignal indicative of the torque applied to the shaft. Optionally, in anembodiment as represented by FIG. 15B, one or more strain-sensitiveelements (e.g., strain gauges) 58 are used to provide a signalindicative of the torque applied to the shaft. The strain-sensitiveelement(s) is(are) preferably located along or orthogonal to theprincipal stress direction for torsional shear, which is oriented at 45°relative to the axis of the shaft. The strain-sensitive element(s) canbe excited either through direct contact or without direct contact(e.g., surface acoustic wave or wireless telemetry). Optionally, thetotal shear strain or angular displacement of the shaft or section ofthe shaft can be measured as a signal indicative of an applied torque tothe shaft.

FIG. 16 is a mechanical isometric view representing a disk or flange asindicated by 70 with centerline 75 undergoing torsional stress andsubsequent strain reapplied at an inner diameter such as from a splineor inner bolt hole circle. The disk is not necessarily a constant crosssection, and may be profiled across its radius as indicated by 78 toprovide a preferred stress distribution across its radius. In thisrepresentative embodiment, a spline 74 is used to apply torque that istransferred radially outwards. In preferred embodiments, the torque istransferred to an adjacent member via a clamping force applied fromfasteners located through a bolt-hole circle indicated by 72, oroptionally splines, gears, or other mechanical features, in which thereaction torque is indicated by 71, such that the disk is undertorsional stress as indicated by 71 and 79. In a preferred embodiment,the disk is magnetized to have one or more bands of remanentmagnetization as indicated by 76, 77. One or more sense-elementsrepresented by 73 located proximate to the disk provide a signalindicative of the torque applied to the shaft. Optionally, one or morestrain sensitive elements (e.g. strain-gauges) can be used to provide asignal indicative of the torque applied to the shaft.

FIG. 17A and B are mechanical isometric views representing the disk orflange of FIG. 16, but indicate the member does not necessarily need tobe continuous about its circumference, as represented by 79, 80 inparticular on embodiments in which the disk or flange is not intended torotate.

9. Further Considerations

Models for hysteresis of magnetic materials are known and welldescribed, such as models to fit standard B-H loops (for example, seeFIG. 6), as well as the application of models and the inverse of modelsapplied to the control of piezoactuators and giant magnetostrictivematerials used for displacement sensors. It was unexpected that thesemodels and methods would have been found to also be applicable to torquetransducers, in particular given that torque transducers have beenobserved to have components of positive and negative hysteresis, whichcomponents are dependent on temperature, applied stress, and the historyof applied stress. It was also originally expected that the influence oftime effects such as eddy currents and reputation, as well as theirpotential dependence upon temperature, would have made the applicationof a corrective algorithm impractical, in particular when applied to areal-time system. However, significant development and testing hassurprisingly shown that when these methods are combined withtemperature-dependent parameters and implemented in real-time, theaccuracy of torque sensor systems can be significantly improved. All ofthe articles and methods disclosed and claimed herein can be made andexecuted without undue experimentation in light of the presentdisclosure. While the articles and methods of this invention have beendescribed in terms of preferred embodiments, it will be apparent tothose of skill in the art that variations may be applied to the articlesand methods without departing from the spirit and scope of theinvention.

All such variations and equivalents apparent to those skilled in theart, whether now existing or later developed, are deemed to be withinthe spirit and scope of the invention as defined by the appended claims.It will also be appreciated that computer-based embodiments of theinstant invention can be implemented using any suitable hardware andsoftware. All patents, patent applications, and publications mentionedin the specification are indicative of the levels of those of ordinaryskill in the art to which the invention pertains. All patents, patentapplications, and publications are herein incorporated by reference intheir entirety for all purposes and to the same extent as if eachindividual publication was specifically and individually indicated to beincorporated by reference in its entirety for any and all purposes.

The invention illustratively described herein suitably may be practicedin the absence of any element(s) not specifically disclosed herein.Thus, for example, in each instance herein any of the terms“comprising”, “consisting essentially of”, and “consisting of” may bereplaced with either of the other two terms. The terms and expressionswhich have been employed are used as terms of description and not oflimitation, and there is no intention that in the use of such terms andexpressions of excluding any equivalents of the features shown anddescribed or portions thereof, but it is recognized that variousmodifications are possible within the scope of the invention claimed.Thus, it should be understood that although the present invention hasbeen specifically disclosed by preferred embodiments and optionalfeatures, modification and variation of the concepts herein disclosedmay be resorted to by those skilled in the art, and that suchmodifications and variations are considered to be within the scope ofthis invention as defined by the appended claims.

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I claim:
 1. A computerized method of real-time hysteresis compensationfor a signal indicative of a sensed torque parameter experienced by amember upon application of a torque or a time-varying torque, the methodcomprising: (a) using a sense element disposed on or in sensing relationto a torque-transmitting member, wherein the sense element generates asignal indicative of a sensed torque parameter from thetorque-transmitting member upon application of a torque or atime-varying torque to the torque-transmitting member, wherein thesignal indicative of the sensed torque parameter exhibits hysteresis;and (b) using a computer comprising a processor, memory, and powersupply to computationally process the signal to compensate in real-timefor the hysteresis in the signal indicative of the sensed torqueparameter, wherein the computer processing the signal indicative of thesensed torque parameter utilizes information of the torque-transmittingmember that is correlated with a torque history or a degree of priorhysteresis compensation for at least one signal indicative of the sensedtorque parameter upon prior application of a torque or a time-varyingtorque to the torque-transmitting member, thereby compensating forhysteresis in the signal in real-time.
 2. A method according to claim 1that further comprises compensating for temperature-related hysteresisvariation.
 3. A method according to claim 1 wherein the sensed torqueparameter is torque or a rate of change of torque applied to the member.4. A method according to claim 1 wherein the member is atorque-transmitting shaft.
 5. A method according to claim 1 wherein themember is a torque-transmitting disk, a torque-transmitting sector of adisk, or a torque-transmitting arm or lever.
 6. A method according toclaim 1 wherein the sense element is disposed proximate to thetorque-transmitting member in order to output a signal indicative of thetorque parameter when the torque-transmitting member experiences or issubjected to a torque or a time-varying torque, wherein optionally thesense element is disposed proximate to the torque-transmitting memberand the output signal measured from the sense element is indicative ofthe torque or time-varying torque transmitted between radially separatedlocations.
 7. A method according to claim 5 wherein the signalindicative of the torque parameter has a hysteresis error of about 0.01%to about 20% for a loading cycle.
 8. An automated, real-timehysteresis-compensating torque measurement system, comprising: (a) atorque-transmitting member; (b) a sense element disposed in sensingrelation to or on a torque-transmitting member, wherein the senseelement is configured to output a signal indicative of a torqueparameter from the torque-transmitting member upon application of atorque or a time-varying torque to the torque-transmitting member,wherein the signal indicative of the sensed torque parameter exhibitshysteresis; (c) a processor operatively associated with the senseelement and configured to (i) process signals output from the senseelement to determine the torque parameter and (ii) compensate inreal-time for hysteresis; (d) a memory operatively associated with theprocessor and configured to store data representing a torque history ordegree of prior magnetic hysteresis compensation in one or more signalsindicative of the sensed torque parameter upon application of a torqueor a time-varying torque to the torque-transmitting member; and (e) apower supply to provide electrical energy for the torque measuringsystem.
 9. A system according to claim 8 wherein the processor isfurther configured to compensate for temperature-related hysteresisvariation in signals output from the sense element.
 10. A systemaccording to claim 8 wherein the sensed torque parameter is torque or arate of change of torque.
 11. A system according to claim 8 wherein thesense element is disposed proximate to a remanently magnetized region ofthe torque-transmitting member in order to output a signal indicative ofa torque parameter when the torque-transmitting member experiences or issubjected to a torque or a time-varying torque.
 12. A system accordingto claim 8 wherein the signal indicative of the sensed torque parameterexhibits a hysteresis error of0. about 01% to about 20% for a loadingcycle.
 13. A system according to claim 8 wherein the torque-transmittingmember is a torque-transmitting shaft.
 14. A system according to claim 8wherein the torque-transmitting member is a torque-transmitting disk, atorque-transmitting sector of a disk, or a torque-transmitting arm orlever.
 15. A system according to claim 8 wherein the sense element ispositioned on the torque-transmitting member in order to output a signalindicative of a torque parameter when the torque-transmitting memberexperiences or is subjected to a torque or a time-varying torque.